It's just a really basic (plain-vanilla, if you will ;) chocolate cake recipe. You'd be better off just finding a basic chocolate cake recipe that calls for unsweetened chocolate in the first place, especially if you're new-ish to chocolate cake.
Otherwise the substitution that Sobachatina suggests above requires some fancy recipe-adjusting work on your part. Cocoa & fat (melted butter, usually) can be substituted for whole chocolate, but it's not really such an exact science, so I don't think the reverse substitution will give you the results you're looking for.
Fructose is one of the sugars in corn syrup. The problem is that 112 C is above the caramelization temperature of fructose, which is 110 C (230 F); this is uniquely low among the various common sugar molecules, most of which begin to caramelize around 160 C (320 F).
The toffee flavor that you are getting is due to the caramelization components.
However, without seeing your exact recipe it is hard to know what to change. I would suggest using table sugar with its much higher caramelization temperature instead of corn syrup would be the most appropriate solution.
It should dissolve the dairy, so you should not need to make a syrup.
However, sucrose (table sugar) has different crystallization properties than fructose and corn syrup, so after you get the main part of the candy mixture up to temperature and then cooled some, you might wish to add a tablespoon or two of corn syrup to help reduce the likelihood of graininess.
You could also try reducing the temperature to say 108 C, but this may affect the final ratio of water to sugar as less water will have been evaporated off, which would change the crystallization pattern in the confection, and thus is texture. It could end up quite sticky or even gooey.
It will probably be easier to search for a recipe for this confection that is already tuned to do what you want, as modifying candy recipes is very tricky; you must get the science and technique just right.
Note: fudge by its very nature is a a solid suspension of very tiny sugar crystals embedded in a dairy/syrup phase. The cooling and beating in traditional fudge recipes are to control the growth of the sugar crystals, so that they are numerous and very, very tiny (which gives the smooth silky texture) as opposed to larger and fewer (which gives a grainy texture).
As fructose and corn syrup tend to resist crystallization, it is odd to have a fudge like recipe with corn syrup as its main sugar component.
Best Answer
Looking at your recipe, the most obvious thing to me is that there is no salt. Adding a small quantity of salt (say, 1/2 tsp) will enhance the flavors of the ingredients already present.
The second thing you might try is switching to dutch processed cocoa; many people find this has a more intense chocolaty taste.
You could try enhancing the overall flavor by adding a small amount of cinnamon (say 1/4 tsp) or instant espresso powder (perhaps 1 tbl). While these ingredients do not, in small amounts, overwhelm the flavor, they do increase its complexity and the impression of how chocolaty the fudge is.
Of course, you can increase the amount of cocoa powder (at least by ratio): simply increase the absolute amount of cocoa, starting with small increments of perhaps 1 tablespoon per test run. The problem with this method is not only that it throws off the sweetness balance, but also that it will eventually change the chemistry, possibly influencing the crystallization of the sugar phase which is what provides the smoothness of the fudge.
Finally, and more radically, consider reducing the amount of dairy, changing the milk for water, or reducing the amount of butter. The milk fats and milk solids tend to mask the flavor of chocolate. While I consider these part of the overall desirable balance of the fudge, since you after an intense chocolate experience, it may be worth experimenting with.